Partial combustion or gasification of coal involves reacting the coal at elevated temperatures and possibly elevated pressures with a limited volume of oxygen, the reaction preferably being carried out in the presence of additional agents such as steam, nitrogen, or various other materials. Gasification of coal produces a gas, known as synthesis gas, that comprises mostly carbon monoxide and hydrogen, Also produced are varying quantities of other gases, such as carbon dioxide, hydrogen sulfide, and methane, and various liquid and solid materials. The small particles of ash and carbon are commonly known and collectively defined herein as flyslag. The flyslag from processes for partial combustion of coal may contain iron, calcium, sulfur, and deposited carbon.
A principal use of the synthesis gas is as a fuel for gas turbines. However, the gas must be extensively purified before it can be used in this manner. In general, the flyslag that is entrained with the gas in partial combustion processes is usually removed from the raw synthesis gas by a combination of cyclones or separators, or a water scrubbing system employing washer coolers, venturi scrubbers, or filters or electrostatic precipitators, or combinations of these systems. In addition to the aforementioned gases, the synthesis gas also contains minor amounts of sulfur-containing gases, such as hydrogen sulfide and carbonyl sulfide, as well as small amounts of ammonia and hydrogen cyanide. The presence of HCN, NH.sub.3, and COS in synthesis gas derived from the gasification of coal complicates removal of additional impurities such as H.sub.2 S, and poses problems insofar as product quality and pollution control requirements are concerned. Although HCN, N.sub.3, and COS are present, as indicated, in quite minor quantities, for example, normally less than 1 percent by volume, combined, of the total raw synthesis gas stream, they must be dealt with before the synthesis gas is utilized.
In one such procedure, residual solids remaining after bulk removal thereof are first removed from the product gas by contacting the gas with an aqueous wash stream. The solids washed from the gas are removed from the wash zone as a thickened stream or slurry and the slurry is sent to a processing zone where the volatiles are stripped and the mixture of liquid and solids remaining is sent to disposal. The synthesis gas, now free of solids, is then sent to an HCN (and possibly COS) removal zone to remove this contaminant, such as by techniques described in U.S. Pat. No. 4,497,784 and U.S. Pat. No. 4,810,475. The synthesis gas stream may then be further cooled and washed to remove virtually all of the ammonia, and gas, now purified of minor impurity compounds, is then ready for treatment to remove H.sub.2 S. The wash water employed, for efficiency, may be utilized in counterflow through the process steps, winding up in the residual solids removal step.
Evidence exists that CO.sub.2 in the synthesis gas may enter the wash water in the ammonia removal step and cause a buildup of calcium carbonate in the scrubbing solution utilized in the residual solids removal zone. It is believed that the CO.sub.2 is dissolved in the wash water in the aqueous wash step because of the high pH that arises from ammonia removal from the synthesis gas, while the calcium enters the system as a leachable component of the residual solids that are scrubbed in the residual solids removal zone. The calcium carbonate deposits are harmful to the chemistry of the system and inhibit heat transfer in the wash zone and the zones to which the slurry is transferred. The invention addresses this problem.